The invention relates to a radio receiver and in particular to a GPS receiver for use with the NAVSTAR Global Positioning System.
The NAVSTAR Global Positioning System is a global, satellite-based radio navigation and time transfer system using the satellite constellation of the US Department of Defence. This constellation will comprise at least 21 satellites positioned in a plurality of orbital paths at a predetermined distance above the Earth and arranged so that, at any time, at substantially any position on the Earth, there will be at least four satellites above the horizon. There is a satellite control station which, inter alia, controls highly accurate clocks in the satellites, synchronises the clocks, determines the orbits of the satellites and uploads orbital information to the satellites for retransmission to users.
Each satellite in the constellation transmits two unique direct-sequence spread spectrum messages in phase quadrature on each of two L-band frequencies. The receiver described here only processes the C/A (Coarse/Acquisition) code message on the L1(1575.42 MHz) carrier; but the principles discussed also apply to the P (Precise) code signals on L1 and L2. The direct sequence code signal, which has a frequency of 1,023 Mchips/sec, and a code epoch repetition rate of 1 kHz, modulates the carrier in a binary phase shift keyed (BPSK) manner. This spread signal is further BPSK modulated by a 50 bps data signal. The data includes information which allows the receiver to measure the range between the receiver and the satellite, i.e. data which allows modelling of the spacecraft orbit (ephemeris) and timing information referenced to the precise satellite clock. The receiver clock will always have an offset with respect to the satellite clock, so these range measurements are known as pseudoranges. To perform a position solution, four pseudorange measurements are required--to solve for the four variables x, y, z and the local clock offset. To further enhance the accuracy of the position solution more measurements can be made--over time (several measurements from the same satellites), over a larger satellite set, or over a wider range of satellite signal variables, such as phase and phase rate.
A position reporting system employing NAVSTAR GPS satellite signals is described in our Co-pending Australian application No. 63995/90.
Many factors influenced the basic design of the GPS receiver of the invention. In addition to particular customer requirements, several design desiderata were identified.
Firstly, the receiver design should be flexible enough to be used in many applications. This implies a modular design with individual modules being interchangeable. In addition, the proportion of the receiver functionality to be provided by software should be optimised, with the software written in modular form, preferably in a high-level language.
Secondly, the receiver design should be suitable for a variety of potential environments. This implies the use of appropriate margins and packaging for the hardware design. More important is the ability of the signal processing to operate under various dynamic conditions, such as those due to vehicle manoeuvring dynamics, or vibration and intermittent blocking of the satellite signals.
Thirdly, a high performance low cost receiver implies the minimisation of production costs which may be achieved by minimizing hardware, either by maximizing the functionality provided by software, or by using a high-density packaging technique, or both. In addition, the receiver is designed to use the C/A code, as opposed to the more complex P code.
Fourthly, using C/A code, a receiver can be almost as accurate as a P code receiver, when it is used in the differential mode. This requires an accurately-positioned base station to provide systematic error corrections of the pseudoranges to the receiver. The receiver must measure the pseudoranges (and any other parameters) to the resolution and accuracy required for the accurate solution, and thus the measurement error and resolution must be much less than the systematic errors which are being removed.
Fifthly, for portability and convenience, the receiver must be designed for minimum mass and volume. In addition, as many proposed applications are for remote, portable use, with power supplied by batteries, power consumption should be minimised. Again, high-density packaging techniques are important.
A known GPS receiver of the closed loop type employs two loops: a code-locked loop which extracts the code delay estimate (pseudorange), and a phase-locked loop (typically a Costas loop) for extraction of the data. The Costas loop may also be used to make ongoing measurements of carrier parameters such as phase and phase rate. In the code-locked loop, a replica of the satellite code is used to despread the received signal, and a data-modulated carrier replica is used to coherently demodulate the despread signal. The resultant energy in each of the early and late channels is balanced in order to align the replica with the received signal. In the Costas loop, the data is demodulated by coherent demodulation.
This phase-locked approach only approximates optimality. Another problem is its susceptibility to loss-of-lock and cycle-slips in low signal to noise ratio (SNR) situations such as in the presence of high noise or vibration or under jamming conditions. The code-locked loop also suffers degraded performance, but is less significant. with respect to total receiver performance. The problem is prevalent in dynamic conditions. However, even in low dynamic conditions such as for high accuracy surveying applications, cycle-slip probability becomes significant since the relevant measurements take long periods of time.
Another type of GPS receiver employs an open loop estimator of signal parameters. In particular, a generic correlation receiver may be used to be the maximum likelihood estimator (ML) of signal parameters. Estimates are made of a selection of signal parameters which are processed to provide a position solution.
The dynamic performance of an open loop design is superior to that of a closed loop design because the former is not subject to the phenomena of cycle slipping and loss of carrier lock. It follows from this that open loop receiver designers have more freedom to vary parameters to meet varying application requirements. Furthermore, an open loop design many be implemented more cost-effectively because of the modularity of the basic processes and because the designer is far less constrained in terms of allowable processing delay.